extensions help: make disabling example analogous to enabling example

The example for enabling an extension in the module search path is:

  foo =

foo can only be disabled by:

  foo = !

and not by:

  hgext.foo = !

As the examples now omit "hgext." the example

  hgext.bar = !

is misleading.

help/extensions.txt

@@ -1,33 +1,33 b''
 Mercurial has the ability to add new features through the use of
 extensions. Extensions may add new commands, add options to
 existing commands, change the default behavior of commands, or
 implement hooks.
 
 Extensions are not loaded by default for a variety of reasons:
 they can increase startup overhead; they may be meant for advanced
 usage only; they may provide potentially dangerous abilities (such
 as letting you destroy or modify history); they might not be ready
 for prime time; or they may alter some usual behaviors of stock
 Mercurial. It is thus up to the user to activate extensions as
 needed.
 
 To enable the "foo" extension, either shipped with Mercurial or in
 the Python search path, create an entry for it in your hgrc, like
 this::
 
   [extensions]
   foo =
 
 You may also specify the full path to an extension::
 
   [extensions]
   myfeature = ~/.hgext/myfeature.py
 
 To explicitly disable an extension enabled in an hgrc of broader
 scope, prepend its path with !::
 
   [extensions]
   # disabling extension bar residing in /path/to/extension/bar.py
-  hgext.bar = !/path/to/extension/bar.py
+  bar = !/path/to/extension/bar.py
   # ditto, but no path was supplied for extension baz
-  hgext.baz = !
+  baz = !